Many printing devices include sensor arrays that are used for evaluating print quality within a printer and to detect print quality defects. Examples of such sensor arrays and related systems are disclosed in U.S. Pat. Nos. 7,309,118 and 7,427,118, the contents of which are incorporated herein by reference. In such systems the sensor array is typically of sufficient size to scan the full width of the imaging device used to impart the test image to a print media.
Image sensors used for scanning images in printing devices typically have a row or linear array of photosensors together with suitable supporting circuitry integrated onto a silicon chip. Usually, a sensor is used to scan line by line across the width of a document or other target with the target being moved or stepped lengthwise in synchronism therewith. One example of a linear sensor array is shown in FIG. 8. FIG. 8 shows various portions of an imaging scanner array 50, in which a substrate 810 has a plurality of silicon chips 812a, 812b, . . . 812z assembled end-to-end and mounted thereon. Also defined on each chip 812a, 812b, . . . 812z is a set of photosensors 814 (also referred to herein as “photo detectors”). These structures may be, by way of example and not limitation, photosensors in a charge coupled device, photogates, CMOS photodiodes, amorphous silicon or transparent electrode MOS type photosites. On each chip 812 there is provided a large number (such as 250 or more) photosensors 814, which are separated by a largely consistent gap between the centers of adjacent photosensors within a chip. The photosensors 814 of the sensor array 50 are substantially aligned along an axis 51 of the sensor array. The photosensors are finely spaced along the axis 51, such as 600 to the inch on each chip.
In a typical printing machine with a sensor array, the plurality of small photosensors extends the full width of an original document or other target, such as a sensor array of 11 inches capable of extending across an 11″×17″ sheet of paper. When the original document or other target moves past the linear array, each of the photosensors converts reflected light from the original image into electrical signals. The motion of the original image perpendicular to the linear array causes a sequence of signals to be output from each photosensor, which can be converted into digital data.
Printing machines configured to accept relatively large sheets of paper typically require longer sensor arrays in order to scan printed images. As sensor arrays become longer, they also become more expensive. One reason for this is because of the increased number of photosensors required in longer sensor arrays. Of course, when more expensive sensor arrays are required, the cost of producing the printing machine is increased.
When the sensor array is used for detecting print quality defects, the sensor array is typically first calibrated against a blank target. Thereafter, a test image is placed on the target with marking material and the sensor array is used to evaluate the print quality of the test image. However, when a sensor array is used for this purpose, there is a risk that contamination may exist on the unmarked calibration target or on the surface of the sensor itself. There is also a possibility that one or more photosensors of the sensor array may be defective during the calibration. Contamination and/or defective photosensors will distort expected readings during calibration such that the sensor readings fall outside of a defined limit. When this happens, a calibration failure results, and the image sensor can not properly evaluate the test image in subsequent scans.
In view of the foregoing, it would be desirable to provide a printing device having a sensor array of reduced size and reduced cost. It would also be advantageous to provide a printing device with such a sensor array where the printing device is capable of identifying a fault location when the sensor array is calibrated in the printing device.